1. Field of the Invention
This invention relates generally to input terminals useful in telautograph systems and, more particularly, to such terminals which provide an electrical indication of the position of a stylus, writing implement or the like with respect to a writing surface.
2. Description of the Prior Art
Numerous telautograph systems, such as the one disclosed in U.S. Pat. No. 3,706,850 issued to G. M. C. Fisher et al. on Dec. 19, 1972, have been devised to communicate graphical information such as handwriting from the point of origination to a distant location. Each such system requires an input terminal that is capable of generating an electrical indication representative of the position of a stylus on a writing or input surface. Briefly described, one type of input terminal that has been proposed includes a resistive sheet and a conductive sheet separated by a small air gap. An energizing potential is applied to the resistive sheet alternately across its "X" and "Y" dimensions, thereby establishing a voltage gradient along each axis. When the resistive sheet is brought in contact with the conductive sheet at a given point under the pressure of a stylus or other writing implement, the voltage output derived from the latter is thus alternately representative of the position of said point along each axis. While this type of input terminal has many advantages over the other apparatus serving a similar function, it also has at least one serious drawback, occasioned by the use of a single resistive sheet to obtain positional information with respect to two axes. Specifically, the energizing potential must be applied to the resistive sheet in a manner in which the points of application along one dimension do not interfere with the points of application along the other dimension. Accordingly, a continuous or strip electrode cannot be affixed to the edges of the resistive sheet, since a strip applied along the X dimension will act as a short circuit during time intervals when the energizing potential is applied across the Y dimension, and vice-versa. Instead, the energizing potential must be applied at discrete electrode points along each axis via diodes or other similar devices that permit conduction in one direction only, and thereby electrically isolate tha nonconducting or "off" electrodes from the energized or "on" electrodes. These diodes, besides adding to the cost and complexity of the input terminal, result in the production of a nonuniform voltage gradient on the resistive sheet, especially near its edges. As a result, the useful writing area of the terminal must be reduced if a truly linear output is desired, or, alternatively, a certain amount of distortion or nonlinearity must be tolerated.
In an attempt to reduce nonlinearity, another type of input terminal known in the prior art utilizes a trilaminar arrangement of first and second resistive layers and a top conductive layer, each separated by a small air gap. An energizing potential is applied alternatively to the resistive layers so as to create a voltage gradient across the X dimension of the first sheet during one time interval and a voltage gradient across the Y dimension of the second sheet during the other time interval. Since two separate resistive layers are used, continuous or strip electrodes may be employed without intersheet interference. However, the trilaminar arrangement is difficult to implement mechanically, and requires a middle layer that is resistive in its thickness dimension as well as across its surface, since the top conductive layer must make electrical contact with the bottom resistive layer when the latter is energized.
With respect to both of the above types of known input terminals, still another problem commonly experienced relates to the generation of stylus-up, stylus-down indications. Specifically, it is desirable to provide an electrical indication at the remote location of whether the stylus is writing or idle. One method of obtaining this indication is to provide a receptacle for the stylus when not in use, the receptacle being arranged to generate a signal when the stylus is lodged therein. Besides being unduly complicated, this approach has been found to be undesirable for the reason that the stylus may not be returned to the receptacle and nevertheless not be in use. Another more desirable method of generating stylus-up, stylus-down signals is to monitor the electrical output of the conductive sheet: when the output attains a value within the prescribed limits of the voltage gradient established across the resistive sheets, the stylus is thus known to be in use. Unfortunately, use of this method may also pose problems, since the resistive sheet or sheets, in combination with the conductive layer, behave like a capacitor, so that the conductive sheet output may remain within the prescribed limits long after the stylus has ceased to be in use, as a result of a lengthy discharge time.
In view of the foregoing, it is the broad object of the present invention to provide an improved graphical input terminal. Specific objects include the design of such a terminal which utilizes only two layers, does not require large numbers of diodes or other unidirectional conduction elements, and which generates distortion free output signals that are completely linear over the entire writing surface. A still further object of the invention is the generation of a stylus-up, stylus-down indication that is reliable and is unaffected by the internal capacitance of the input terminal.